Radiosurgery and radiotherapy systems are radiation therapy treatment systems that use external radiation beams to treat pathological anatomies (tumors, lesions, vascular malformations, nerve disorders, etc.) by delivering prescribed doses of radiation (X-rays, gamma rays, electrons, protons, and/or ions) to the pathological anatomy while minimizing radiation exposure to the surrounding tissue and critical anatomical structures. Radiotherapy is characterized by a low radiation dose per fraction (e.g., 100-200 centiGray), shorter fraction times (e.g., 10-30 minutes per treatment), and hyper fractionation (e.g., 30-45 fractions), and repeated treatments. Radiosurgery is characterized by a relatively high radiation dose per fraction (e.g., 500-2000 centiGray), extended treatment times per fraction (e.g., 30-60 minutes per treatment), and hypo-fractionation (e.g. 1-5 fractions or treatment days). Due to the high radiation dose delivered to the patient during radiosurgery, radiosurgery requires high spatial accuracy to ensure that the tumor or abnormality (i.e., the target) receives the prescribed dose while the surrounding normal tissue is spared.
In general, radiosurgery and radiotherapy treatments consist of several phases. First, a precise three-dimensional (3D) map of the anatomical structures in the area of interest (head, body, etc.) is constructed using any one of a computed tomography (CT), cone-beam CBCT, magnetic resonance imaging (MRI), positron emission tomography (PET), 3D rotational angiography (3DRA), or ultrasound techniques. This determines the exact coordinates of the target within the anatomical structure, namely, locates the tumor or abnormality within the body and defines its exact shape and size. Second, a motion path for the radiation beam is computed to deliver a dose distribution that the surgeon finds acceptable, taking into account a variety of medical constraints. During this phase, a team of specialists develop a treatment plan using special computer software to optimally irradiate the tumor and minimize dose to the surrounding normal tissue by designing beams of radiation to converge on the target area from different angles and planes. Third, the radiation treatment plan is executed. During this phase, the radiation dose is delivered to the patient according to the prescribed treatment plan.
There are many factors that can contribute to differences between the prescribed radiation dose distribution and the actual dose delivered (i.e., the actual dose delivered to the target during the radiation treatment). One such factor is uncertainty in the patient's position in the radiation therapy system. Other factors involve uncertainty that is introduced by changes that can occur during the course of the patient's treatment. Such changes can include random errors, such as small differences in a patient's setup position. Other sources are attributable to physiological changes that might occur if a patient's tumor regresses or if the patient loses weight during therapy. Another category of uncertainty includes motion. Motion can potentially overlap with either of the categories as some motion might be more random and unpredictable, whereas other motion can be more regular. These uncertainties can affect the quality of a patient's treatment and the actual radiation dose delivered to the target.
The accuracy in delivering a predicted radiation dose to a target based on a predetermined treatment plan, therefore, plays an important role in the ultimate success or failure of the radiation treatment. Inaccurate dose delivery can result in either insufficient radiation for cure, or excessive radiation to nearby healthy tissue. Quality assurance tools and protocols are therefore needed to verify that the prescribed radiation dose is delivered to the target.
In radiation therapies, radiation delivery is made based on the assumption that the radiation treatment plan was developed based on correct information, the position of the radiation beam relative to the patient set-up is correctly calibrated, and that the radiation therapy system not only functions properly but that it also functions based on correct and consistent external inputs used to program the system. However, if the calibration of the support device, for example, is incorrect, or the system functions improperly, or the treatment plan includes incorrect information, an incorrect dose will be delivered to the target during treatment even if the radiation therapy system operates as instructed. A radiation dose that is too high may cause serious damage to healthy tissues surrounding the tumor, whereas a dose that is too low may jeopardize the probability of cure. Therefore, a relatively small error in the delivered radiation dose may seriously harm the patient.